Hybrid electric vehicle with reduced auxiliary power to batteries during regenerative braking

ABSTRACT

An electric vehicle is controlled to conform its operation to that of a conventional internal-combustion-engine powered vehicle. In some embodiments, the charging of the batteries by the auxiliary source of electricity and from dynamic braking is ramped in magnitude when the batteries lie in a state of charge between partial charge and full charge, with the magnitude of the charging being related to the relative state of charge of the battery. The deficiency between traction motor demand and the energy available from the auxiliary electrical source is provided from the batteries in an amount which depends upon the state of the batteries, so that the full amount of the deficiency is provided when the batteries are near full charge, and little or no energy is provided by the batteries when they are near a discharged condition. At charge states of the batteries between near-full-charge and near-full-discharge, the batteries supply an amount of energy which depends monotonically upon the charge state. Charging of the batteries from the auxiliary source is reduced during dynamic braking when the batteries are near full charge. Control of the amount of energy returned during dynamic braking may be performed by control of the transducing efficiency of the traction motor operated as a generator.

This patent application claims priority of Provisional patentapplication Ser. No. 60/066,736, filed Nov. 21, 1997.

FIELD OF THE INVENTION

This invention relates to apparatus and method for making the operationand operating characteristics of hybrid electric vehicles simple andeffective.

BACKGROUND OF THE INVENTION

Hybrid electric vehicles are widely viewed as being among the mostpractical of the low-polluting vehicles. A hybrid electric vehicleincludes an electric "traction" battery which provides electric powerfor an electric traction motor, which in turn drives the wheels of thevehicle. The "hybrid" aspect of a hybrid electric vehicle lies in theuse of a secondary or supplemental source of electrical energy forrecharging the traction battery during operation of the vehicle. Thissecondary source of electrical energy may be solar panels, a fuel cell,a generator driven by an internal combustion engine, or generally anyother source of electrical energy. When an internal combustion engine isused as the secondary source of electrical power, it commonly is arelatively small engine which uses little fuel, and produces littlepollution. A concomitant advantage is that such a small internalcombustion engine can be operated within a limited RPM range, so thatpollution controls of the engine may be optimized. The terms "primary"and "secondary" when used to describe the sources of electrical energymerely relate to the way energy is distributed during operation, and arenot of fundamental importance to the invention. A simple electricallydriven vehicle powered only by electrical batteries has thedisadvantages that the batteries may become depleted while the vehicleis far from a battery charging station, and even when such a vehiclesuccessfully returns to its depot after a day's use, the batteries mustthen be recharged. The hybrid electric vehicle has the significantadvantage over a simple electrically powered vehicle that the hybridelectric vehicle recharges its own batteries during operation, and soshould not ordinarily require any external battery charging. Thus, thehybrid electric vehicle can be used much like an ordinary vehiclepowered by internal combustion engines, requiring only replenishing ofthe fuel. Another major advantage of the hybrid electric vehicle is itsgood fuel mileage. The advantage in fuel mileage arises from the use ofregenerative dynamic braking, which converts kinetic energy of motioninto electrical power during at least a portion of braking, and returnsthe energy to the battery. It has been found that braking losses accountfor somewhere near half of all the frictional losses experienced by avehicle in an urban transit setting. The recovery of this 50% of energy,and returning it to the batteries for further use, permits the use of amuch smaller "secondary" fuel-operated electrical generator than wouldbe the case if regenerative braking were not used. In turn, the smallersecondary electrical source results in less fuel used per unit time, orper mile. Yet another advantage of a hybrid electric vehicle is thatunder many conditions, the power which is available for accelerating thevehicle is the sum of the maximum power which can be supplied by thebatteries plus the maximum power which can be generated by the secondaryelectrical generator. When the electrical generator is a diesel-poweredinternal combustion engine, the combination of the battery power and thediesel power can result in a total motive force which is quitesubstantial, notwithstanding the good fuel mileage.

While hybrid electric vehicles are economically and environmentallyadvantageous, they must be somewhat "foolproof", in that they must besimilar to conventional internal-combustion-powered vehicles, in theiroperation and in their responses to operator input, in order to achievewidespread acceptance.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a method for operating a hybridelectric vehicle which derives at least some of its tractive effort fromelectric batteries includes the step of providing energy from a tractionbattery to a traction motor in at least one operating mode of the hybridelectric vehicle, and, from time to time, dynamically braking thevehicle. The steps of the method include returning to the batteries atleast a portion of the energy made available by the dynamic braking, andcharging the batteries from an auxiliary source of electrical powerduring those intervals in which the dynamic braking is not performed,with the charging being in a "normal" amount, which may vary accordingto some control law, in a manner suitable for normal operation of thevehicle. In this aspect of the invention, the batteries are charged fromthe auxiliary source of electrical power during those intervals in whichdynamic braking is being performed, at a rate reduced from the "normal"amount of charging. In another improvement of this aspect of theinvention, the step of charging the batteries from an auxiliary sourceof electrical power includes the step of charging the batteries from anelectrical generator driven by an internal combustion engine, which maybe a diesel engine. A fuel cell can be used instead of anengine-generator combination.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified block diagram of an electric vehicle according toan aspect of the invention, including a command controller whichperforms control in accordance with the invention, and also including apower controller;

FIG. 2 is a simplified block diagram illustrating some of the functionsperformed within the power controller of FIG. 1;

FIGS. 3a and 3b are simplified plots of energy regeneration to thetraction battery versus traction battery charge state and traction dueto regeneration versus traction battery charge state, respectively;

FIG. 4 is a simplified flow chart illustrating the logic flow in thecommand controller of FIGS. 1 and 2 to provide the operationsillustrated in FIGS. 3a and 3b;

FIG. 5 illustrates a simplified plot of the distribution of the supplyof traction power to the traction motor of the vehicle of FIG. 1 as afunction of traction battery charge:

FIG. 6 is a simplified flow chart illustrating the logic flow in thecommand controller of FIGS. 1 and 2 to provide the operationsillustrated in FIG. 5;

FIG. 7a is a plot of motor or generator power versus speed with torqueas a parameter, and FIG. 7b is a representation of how the power of themotor/generator is controlled; and

FIG. 8 is a simplified block diagram illustrating certain controlcircuits or arrangements for controlling the amount of electric powergenerated by the auxiliary power source in response to the state ofcharge of the traction battery.

DESCRIPTION OF THE INVENTION

In FIG. 1, an electric vehicle 10 includes at least one drive wheel 12connected to an alternating voltage electric traction motor 40, which inone embodiment of the invention is a three-phase alternating-currentmotor. Motor 40 is preferably a motor-generator, as known, so thatkinetic energy of motion can be transduced into electrical energy duringdynamic braking. A power controller 14 is connected by power-handlingpaths to traction motor 40, to a traction battery illustrated as 20, andto an auxiliary source of electrical energy illustrated as a block 16.As illustrated in block 16, the auxiliary source may include an internalcombustion engine such as a diesel engine 18 driving an electricalgenerator 22, or it may include a fuel cell 24. A command controllerillustrated as a block 50 is connected by means of information paths topower controller 14, auxiliary source 16, and to traction motor 40, forcontrolling the operation of the power controller 14, auxiliary source16, and to traction motor 40 in accordance with appropriate controllaws.

One of the most common and least expensive types of batteries which iscapable of storing relatively high power includes the common lead/H₂ SO₄battery. This type of battery is suitable for use in an electricvehicle, if some care is taken to prevent application of a chargingcurrent thereto when the battery is at full charge, to prevent gassingof the electrolyte and undesired heat generation, and if sulfation canbe avoided. Two copending patent applications Ser. Nos. 08/961,571 and08/961,573, filed Oct. 30, 1997 in the name of Hoffman, Jr. and Grewe,and entitled Method for Equalizing the Voltage of Traction BatteryModules of a Hybrid Electric Vehicle and Method for Maintaining theCharge Capacity of Traction Battery Modules of a Hybrid ElectricVehicle, respectively, describe control arrangements by which lead-acidbatteries can be maintained to optimize their useful life and capacity,and describe various aspects of the care and use of such batteries.

In FIG. 1, the displays and operator controls of vehicle 10 areillustrated as a block 30. Block 30 is illustrated as being connected bya bidirectional data path 31 to command control block 50, for applyingdriving commands to command controller 50, which command controller 50can then convert into appropriate commands to the various powerelements, such as power controller 14, auxiliary source 16, and tractionmotor 40. Block 30 is also illustrated as being connected by a path 32to friction brakes 36a and 36b, for direct control of the frictionbrakes by a conventional hydraulic braking system connected to a brakepedal.

FIG. 2 represents the interconnection of some of the elements of powercontroller 14 of FIG. 1 with other elements of FIG. 1. Moreparticularly, power controller 14 includes a rectifier arrangement 26connected to auxiliary source 16, for (if necessary) convertingalternating-current output of the auxiliary source 16 into directvoltage. Power controller 14 also includes a bidirectional propulsioncontrol system, which further includes a dc-to-ac inverter 28 coupled bypower connections to battery 20, to rectifier arrangement 26, and totraction motor 40. The operations of the inverter 28, the auxiliarysource 16, and traction motor 40 are controlled, as mentioned above, bycommand controller 50. It should be noted that in addition to thedc-to-ac inverter 28, the propulsion control system includes voltage andcurrent sensors, to sense the various operating parameters of themotor/generator, battery, and auxiliary electrical source.

In basic operation of the arrangement of FIGS. 1 and 2, the commandcontroller (50) controls the individual switches (not illustrated) ofinverter 28 with pulse-width-modulated commands, which result in thegeneration, at that port 28m of the inverter 28 which is coupled to thetraction motor 40, of an approximation of an alternating voltage havinga selected frequency and magnitude. In a preferred embodiment of theinvention, the inverter is a field oriented command (FOC) type, andtraction motor is similarly an FOC induction motor. The frequency andmagnitude of the commanded alternating current drive to the tractionmotor 40 are selected to drive the motor with a selected tractioncurrent at a selected motor speed. In general, traction motor 40produces a back EMF which increases with increasing motor speed, and theinverter must produce (under the control of command controller 50) analternating voltage which increases in magnitude with increasingalternating voltage frequency in order to maintain the same tractionmotor drive current. The motor rotates at a frequency consistent withthe commanded frequency of the inverter output. Also in basic operationof an electric vehicle such as that of FIGS. 1 and 2, both dynamicbraking and friction braking may be performed. The dynamic braking ismuch preferred, as the (kinetic) energy inherent in the motion of thevehicle is recaptured, by the traction motor operating as an electricgenerator, as the vehicle is slowed. During those intervals in whichdynamic braking occurs, the dc-to-ac inverter 28 of FIG. 2, operating ina second or regenerating direction, converts the alternating voltageproduced by the traction motor 40 into a direct voltage which chargestraction battery 20. Further, when the electric vehicle is a hybridelectric vehicle, including the auxiliary electric source 16, theauxiliary source can be operated during operation of the vehicle toreplenish the batteries andor to provide some of the traction energy,depending upon the commands of command controller 50.

It has been noticed that, when an electric vehicle is operated in anormal mode using dynamic braking, and the batteries are fully charged,the dynamic braking tends to push a charge current through thealready-charged battery. The characteristics of a lead-acid battery aresuch that, in this situation of applying a charging current to a fullycharged battery, the battery voltage tends to rise markedly, as from afully charged, no-current value of 13 volts, in a nominally 12-voltbattery, to somewhere near 16 volts, thereby providing an indication tothe command controller that an overcharging condition is occurring. Ifthe command controller decouples the energy generated by dynamic brakingfrom the battery, as it must in order to protect the battery, thebattery voltage immediately drops to its fully-charged no-current value.This, in turn, allows the dynamic braking controller to once again beginto provide energy to the battery until the overvoltage control takeseffect. This results in periodic application of the dynamic braking at apulse rate established by the loop characteristics of the commandcontroller, and produces a perceivable braking chatter, as well astending to overcharge the battery during portions of the pulse interval.Both the overcharging and the chatter are undesirable.

FIGS. 3a and 3b together illustrate a control law according to an aspectof the invention, which allows full regeneration or return to thetraction batteries of energy derived from dynamic braking during thoseintervals in which the traction batteries are at a state of charge lessthan a particular amount of charge, which particular amount of charge isless than full charge, and which, at traction battery charge levelslying between the particular charge and full charge, tapers theproportion of the regenerated energy derived from dynamic braking in amanner which is responsive or a function of the then-existing state ofcharge relative to the difference in charge between the predeterminedcharge and full charge. In one embodiment of the invention, therelationship is monotonic, and the relationship may be linear. In FIG.3a, plot 310 represents the amount of regeneration as a function oftraction battery charge state pursuant to a control law in accordancewith an aspect of the invention. More particularly, plot 310 defines aportion 312 which is constant at a value of dynamic braking regenerationwhich represents 100% regeneration, or as close to 100% as isconveniently possible. At full charge, the amount of regeneration of theenergy derived from dynamic braking is reduced to near zero, or as closeto zero as is conveniently possible. The control law represented by plot310 further includes a second portion 314, which ramps monotonicallyfrom 100% regeneration at a predetermined traction battery charge leveldenominated "first charge" to zero regeneration at full charge of thetraction battery. The effect on the regenerative traction or braking ofthe vehicle as a function of traction battery charge condition isillustrated by a plot 320 in FIG. 3b. In FIG. 3b, plot 320 includes afirst portion 322, which extends at a constant value representingmaximum regenerative traction from low charge levels to the "first"level of charge of the traction battery. A second portion 324 of plot320 represents regenerative traction which ramps monotonically from 100%at the "first" charge level to 0% at full charge. While the portions 314and 324 of plots 310 and 320, respectively, are illustrated as linearramps, it is sufficient for control purposes that the portions 314 and324 be monotonic. This monotonic reduction in dynamic braking should notbe perceptible to the driver of the automobile, since the charge stateof the traction battery changes slowly, and therefore the amount ofregenerative braking changes slowly. Since the regenerative brakingchanges slowly, the friction brakes gradually take up any deficitbetween the dynamic braking and the desired braking force. This, inturn, should reduce the chatter which is evident when the control lawsimply protects the traction battery from overcharge by simply stoppingthe regeneration when the batteries are at full charge.

FIG. 4 is a simplified flow chart illustrating that portion 400 of thecontrol laws controlling the control processor 50 of FIG. 1 whichresults in the type of performance represented by FIGS. 3a and 3b. InFIG. 4, the logic starts at a START block 410, and proceeds to a block412, which represents monitoring the traction battery pack (20 ofFIG. 1) parameters such as temperature, voltage, and current, and alsonoting time. Samples of these parameters may be taken at frequentsampling intervals, such as at each iteration of the logic through theloop of FIG. 4. From logic block 412, the logic flows to a block 414,which represents an estimation of the state of charge of the tractionbattery, by determining the amount of charge which has entered thebattery, and subtracting the amount of charge which has left thebattery. The measure of this charge is the amphour. Once an estimate ismade of the state of charge of the traction battery, the logic flows toa decision block 416, which compares the current or present-timeestimated state of charge of the traction battery with the predeterminedvalue of charge represented by the "first charge" level of FIGS. 3a and3b; as mentioned above, this charge level is less than full charge. Ifdecision block 416 finds that the estimated charge level of the tractionbattery is less than the first charge level, the logic leaves decisionblock 416 by the YES output, and proceeds to a further block 418, whichrepresents allowing full regenerative braking energy or power to beutilized. The action taken in block 418 may be, for example, adjustingthe field current in the traction motor (operating in its generatormode) during braking so as to maximize the electrical output of thetraction motor. It should be noted that some types of motor/generatorshave no distinct field winding, but rather have pluralities of windingsin which one winding has its desired current induced or inducted bycontrolled current in another winding; for purposes of the invention,the way the field current is generated is irrelevant, it is sufficientthat it is generated in the desired amount. From block 418, the logicflows back to block 412 to begin another iteration around the loop. Asthe hybrid electric vehicle is driven in this state, the tractionbattery will often become more fully charged due to the continuousinjection of energy (by the action of the auxiliaryinternal-combustion-engine/generator) into the energy storage systemwhich includes the traction battery and the motion of the vehicle.

Eventually, the state of charge of the traction battery will exceed the"first charge" level illustrated in FIGS. 3a and 3b. At that time, theiterations of the logic of controller 50 of FIG. 1 around the portion ofits preprogrammed logic represented by logic loop 400 of FIG. 4 willchange, since the logic flow will no longer be directed from the YESoutput of decision block 416, but will instead be directed to the NOoutput. From the NO output of decision block 416, the logic flows to afurther block 420, which represents reduction of the magnitude of theregenerative power or energy available in the form of kinetic energy ofthe vehicle, in inverse relation or proportion to the present-timeamount of charge relative to the difference between full charge and thefirst charge level of FIGS. 3a and 3b. Thus, if the current state ofcharge is at 70% of the way between the first charge and full charge, asillustrated by C_(c) in FIGS. 3a and 3b, the amount of the energy ofmotion which is allowed to be recovered and coupled to the battery is30%. When the current charge level reaches 100%, the allowableregeneration is 0%. As mentioned above, the control of coupling ofenergy or power from the traction motor acting as a generator can beaccomplished simply by adjusting the command torque of the drive in afield oriented controlled alternating current motor. In an actualembodiment of the invention, the torque is reduced proportionally tospeed in order to control the amount of power produced by the motoracting as a generator which is returned to the traction battery.

As so far described, the logic of FIG. 4 controls the regeneration inaccordance with the state of charge of the traction battery. This meansthat the retarding force acting on the vehicle by the traction motoracting as a generator is reduced during braking. One of the advantagesof an electric vehicle which uses regenerative braking is that thefriction brakes are not required to do all of the braking, and so theirdesign and construction may be such as to take advantage of the lesserusage, as for example by making them lighter in construction. As so fardescribed in conjunction with the logic of FIG. 4, the dynamic brakingis reduced under certain charge conditions of the traction battery. Inorder to provide additional braking during those times when theregenerative braking is reduced, according to another aspect of theinvention, the logic flows from block 420 of FIG. 4 to a further block422, which represents reduction of the efficiency of the traction motoracting as a generator. This reduction of the efficiency of the tractionmotor acting as a generator can be accomplished by adjustment of eitherthe slip or of the current in the field winding, or preferably both.From block 422 of FIG. 4, the logic returns to block 412, to beginanother iteration "around the loop" or through the logic 400.

As so far described, the chatter or uneven performance resulted fromprotection of the fully-charged battery from additional charge. Asimilar effect occurs upon acceleration with a nearly dischargedbattery. During acceleration of the vehicle 10 of FIG. 1, both thetraction battery 20 and the auxiliary or secondary electrical source 16(the internal-combustion-engine/generator) are available as sources ofelectrical energy for the traction motor 40. Consequently, the tractionmotor 40 can provide power at a rate which is the sum of the maximumpower which can be drawn from the traction battery 20 together with themaximum power which the auxiliary source 16 can provide. This isconvenient for operation in a city, where bursts of acceleration mayrequire significant power. However, under some conditions, the tractionbattery protection controls, if they simply stop drawing power from thetraction battery when the battery reaches a state of charge which isdeemed to be a discharged state, will also cause a form of chatter. Thisform of chatter occurs if the vehicle is running uphill for a longperiod of time, such as in crossing the Continental Divide. If the rateof utilization of energy in raising the vehicle along the road exceedsthe rate of delivery of energy by the auxiliary source 16, the batterieswill continuously discharge, and eventually reach the level of chargedeemed to be the "discharged" level. If, at that time, the tractionbattery controller were to simply cut the traction battery from thetraction motor circuit, the amount of current available to the tractionmotor would suddenly decrease to the level provided by the auxiliarysource 16, with a consequent abrupt change in tractive power, and thevehicle would experience a sudden reduction in speed. Removal of thetraction battery discharge to the traction motor, however, allows thebattery voltage to rise abruptly to its no-load voltage. If thecontroller interprets this rise in voltage as indicating that thetraction battery has usable charge, it may reconnect the tractionbattery to the traction motor, thereby once again providing additionaltractive power from the traction battery, but causing the voltage of thetraction battery to drop. Those skilled in the art will recognize thisas an oscillatory condition, which may cause the vehicle to "chug" orlurch repeatedly during the climb.

It should be noted at this point that a "fully" discharged battery, inthe context of a traction battery in which long life is desired, stillcontains a substantial charge, because the life of such batteries isdramatically reduced if the depth of discharge is too great; thus adischarge battery for the purposes of discussion of electrically drivenvehicles is one in which the batteries are at a state of charge which isdeemed to be the full-discharged condition, but which still contains asubstantial charge. In a hybrid electric vehicle, the auxiliary energysource provides energy continuously, which can be used to charge thetraction batteries if the traction demand is less than the output of theauxiliary energy source. The control laws allow both the auxiliaryenergy source and the traction batteries to provide energy to thetraction motor. When traction motor demand exceeds auxiliary sourceoutput, current is drawn from the traction battery, which causes itsvoltage to drop. If the traction battery is near a full dischargecondition, the voltage drop due to this current draw may be such as totrigger battery protection by stopping the current drain from thebattery. The removal of the current drain by the control laws, in turn,causes the vehicle to be powered solely by the auxiliary source, andallows the voltage of the traction battery to rise. When the tractionbattery rises, the control laws no longer recognize the battery as beingdischarged, and current drain is again allowed from the tractionbattery. The process of repeatedly coupling and decoupling the tractionbattery to the traction motor constitutes an oscillation of the controlsystem. This oscillation results in a traction force which varies at thecontrol system oscillation rate, and which may be perceptible to theoperator of the vehicle.

According to another aspect of the invention, controller 50 controls theamount of power which can be drawn from the traction battery in responseto the state of charge of the traction battery. This avoids theabovedescribed "chugging" situation, and allows a smooth decrease in thespeed with which the vehicle can climb a mountain as the battery chargedecreases. FIG. 5 illustrates a plot 500 which represents the result ofcontrol in accordance with this aspect of the invention. In FIG. 5,traction power available to the vehicle is plotted against the state orlevel of charge of the traction battery. Plot 500 includes a portion510, which represents the continuous output of the auxiliary source ofelectrical energy or power, which is a relatively low level. Plotportion 510 extends from a level less than the nominal dischargecondition to a charge level designated as "low charge point," which isthe nominal discharged condition of the traction battery. In anoperating region represented by plot portion 512, the tractive poweravailable to the vehicle is at a relatively high level, representing thesum of battery and auxiliary power. This maximum power level representedby plot portion 512 extends from a charge condition denominated as"first charge" to the fully-charged condition. Between the "low charge"condition of the traction battery and the "first charge" condition, theamount of tractive power depends upon the state of charge of thetraction battery, as suggested by plot portion 514. The effect of thistype of control is to allow operation at full tractive power for aperiod of time, until the traction battery is partially discharged tothe "first" level. As the traction battery drops just below the firstlevel, the amount of battery power which is available to the tractionmotor is decreased slightly, in an amount which is hoped is notnoticeable. This slight decrease in power at a point just below thefirst charge level of FIG. 5 somewhat reduces the rate of discharge ofthe traction battery. If the hill is long, the traction battery maydischarge further. As the traction battery becomes further discharged inthe region between the "low" and "first" charge condition of FIG. 5,relatively less of the battery power is made available to the tractionmotor, resulting in a further slowing of the vehicle. For the longesthills, the traction battery will ultimately reach the "low" chargecondition which is deemed to be nominally discharged. When this level isreached, no more energy is extracted from the traction battery, and, ingeneral, the state of charge of the traction battery cannot extend belowthe "low" charge level into plot portion 510, unless there is some otherdrain on the traction battery, such as an emergency override of batteryprotection under conditions of imminent danger to the vehicle or itsoccupants. With control as plotted in FIG. 5, there is no abrupttransition in tractive power at any point along the control curve. Whenthe battery charge is just above the "low" charge point, and is makingthe transition to full operation from the auxiliary electrical source,the amount of tractive power provided by the traction battery is alreadyvery small, and the transition should be imperceptible to the vehicledriver.

FIG. 6 is a simplified flow chart which illustrates that portion 600 ofthe logic of controller 50 of FIG. 1 which provides the control inaccordance with plot 500 of FIG. 5. In FIG. 6, the logic begins at aSTART block 610, and proceeds to a block 612, which represents readingof the battery characteristics, much as in block 412 of FIG. 4. Fromblock 512 of FIG. 5, the logic flows to a block 614, which representsestimation of the state of charge, also as described generally in FIG.4. Decision block 616 of FIG. 6 determines if the current state ofcharge is above the "first" charge point of FIG. 5, and routes the logicby way of the YES output of decision block 616 if the charge state isgreater than the "first" charge point. From the YES output of decisionblock 616, the logic flows to a block 618, which represents the makingof full traction power available to the traction motor. This isaccomplished by removing power limits, as described in conjunction withFIGS. 7a and 7b, in the software controlling the inverter, noting thatthe auxiliary source is a source only, while the battery and themotor/generator can be sources or sinks, depending on the operation ofthe inverter. From block 618, the logic flows back to block 612, tobegin another iteration through the logic of FIG. 6. In general, whenstarting out with a near-fully charged traction battery, the logic williterate around the loop including blocks 612, 614, 616, and 618 of FIG.6 for so long as the traction battery charge exceeds the chargerepresented by the "first" charge level in FIG. 5.

On a long climb, the traction battery charge may eventually drop toequal or less than the "first" charge point of FIG. 5, and on the nextiteration through the logic of FIG. 6, the logic 6 will exit decisionblock 616 by the NO output, and will proceed to a block 620. Block 620represents reduction in the amount of power available to the tractionmotor from the traction battery in an amount which depends upon themagnitude of the current traction battery charge relative to thedifference in charge between the "first" and "low" charge states of FIG.5. For example, if the present-time level of charge of the tractionbattery drops below the "first" charge condition of FIG. 5 to a levelrepresented in FIG. 5 as "current charge," which is 9/10 of the waybetween the charge levels represented by the "low" and "first" chargelevels, controller 50 controls the amount of power available to thetraction motor from the traction battery to be 90% of thebattery-supplied component of the full power represented by plot portion512. Put another way, since the current state of charge indicated inFIG. 5 as "current charge" is 90% of that component of the full tractionpower designated as being attributable to the battery, the battery powerprovided to the traction motor is reduced to 90% of the battery power.Naturally, there is no requirement that plot portion 514 of FIG. 5 be alinear ramp as illustrated, but the control system is simplified if plotportion 514 is at least monotonic. From block 620 of FIG. 6, the logicflows to a decision block 622, which compares the traction motor powerdemand with the power from the auxiliary source of electrical energy. Ifthe traction power demand exceeds the power from the auxiliary source ofelectricity, the batteries are being discharged, and the logic leavesdecision block 622 by the YES output. From the YES output of decisionblock 622, the logic flows to a block 624, which represents increasingthe power available from the auxiliary source to its maximum value. Fromblock 624, the logic flows to a decision block 626. Decision block 626compares the current state of charge of the traction battery with the"low" charge point of FIG. 5. If the state of charge is below the "low"charge point, indicating that the traction battery should not be furtherdischarged in order to prevent damage to the traction battery, the logicleaves decision block 626 by the YES output, and proceeds to a logicblock 628. Block 628 represents limitation of the traction motor power,by FOC control, to the known amount of power available from theauxiliary source of electrical energy, readily determined as the productof the voltage multiplied by the current. From block 628, the logicflows by way of a logic path 630 back to block 612 by way of a logicpath 630, to begin another iteration through the logic of FIG. 6. If,when decision block 626 examines the state of charge of the tractionbattery, the current state of charge is greater than the "low" chargepoint of FIG. 5, the logic leaves decision block 626 by the NO output,and proceeds over logic path 630 back to block 612, without transitingblock 628. Thus, when there is significant usable charge in the tractionbattery, the logic of FIG. 6 permits its use. If, during the transit ofthe logic through FIG. 6, decision block 622 finds that the tractionpower is not greater than the power produced by auxiliary source 16, thelogic leaves decision block 622 by the NO output, and proceeds by way oflogic path 630 to block 612, to begin another iteration; this pathbypasses the increasing of the power of the auxiliary source 16 to themaximum.

FIG. 7a illustrates a simplified parametric plot 710a, 710b, 710c, . . ., 710N of motor (or generator) power versus speed. In FIG. 7a, plots710a, 710b, 710c, . . . , 710N have a sloped portion 712 in common.Power for a motor or generator is the product of torque multiplied byspeed. Consequently, at zero speed, the power is zero, regardless of thetorque. As speed increases at constant torque, the power increases, assuggested by portion 712 of the plots of FIG. 7a, up to a speedω_(base). Above frequencies of ω_(base), the design of themotor/generator is such that no more power can be handled, for thermalor other reasons. Consequently, at maximum torque, the power of themotor/generator is limited by the control laws of the inverter to lie onplot 710a. If the torque is somewhat less than the maximum torque, themaximum power is achieved at a slightly lower motor speed than omega subbase, represented by plot 710b. Plot 710c represents a still lowermagnitude of torque, and the lowermost plot, 710N, represents the lowesttorque which the quantized control system can sustain. The controlsystem will limit the torque produced by the motor to a limiting value,depending upon the speed, to prevent the motor from operating at abovethe desired maximum power limits. The limiting torque₋₋ limit isdetermined simply by dividing the maximum power by the current motorspeed torque₋₋ limit=P_(max) /speed and the resulting limit on torquecauses the power plot to limit at a value no greater than thatrepresented in FIG. 7a by plot 710a and plot portion 712. If the poweris to be limited to a lesser value than P_(max), the power plot whichthe motor follows will correspond to one of plots 710b, 710c, . . . ,710N of FIG. 7a. FIG. 7b is a simplified block diagram illustrating therelationship of the torque command and the power limiter. In FIG. 7b,the torque₋₋ command is applied to a limiter block 714, which adjuststhe magnitude of the torque command (Limited Torque₋₋ Cmd) which arrivesat the Field Oriented Control (FOC) inverter 28 in a manner which limitsthe power to lie under a curve 716. Curve 716 is a plot of torque versusspeed determined by dividing the selected or set power P by the motorspeed. Thus, the FOC inverter can control the power of the motor bycontrol of the commanded torque in view of the motor speed. The torquein question may be traction or driving torque, or it may be retarding orbraking torque. When control of the power flowing to the batteries fromthe motor, acting as a generator, is desired, the appropriate FOCcommands result in application of the limit.

In FIG. 8, the desired torque or torque command is derived from anelectrical accelerator (not illustrated) and applied by way of a path810 to a first input port of a multiplier 812, which receives sensedvehicle speed (or traction motor speed if the vehicle is equipped withchangeable gears) from sensors (not illustrated) at its second inputport 814. Multiplier 812 takes the product of motor speed and commandedtorque, to produce a signal representing commanded power to be appliedto the traction motor. A block 816 scales the commanded power by aconstant k, if necessary, to convert the signal to a representationP_(c) of commanded traction motor power in watts. The signal P_(c)representing the commanded power in watts is applied from block 816 to afurther block 818, which represents the division of the commanded powerin watts by the traction battery voltage, to get a signal representingthe commanded traction motor current (I_(c) =P/E). The traction batteryvoltage is an acceptable indicator of the traction motor voltage,because all the voltages in the system tend toward the battery voltage.The signal representing the commanded current I_(c) is carried by asignal path 819 to a portion of the command controller 50 of FIG. 1 forcontrol of the FOC inverter 28 and the traction motor 40 in a mannerwhich produces the desired motor current. the signal representing thecommanded current I_(c) is also applied from the output of block 818 byway of a scaling circuit illustrated as a block 820 to an error signalgenerator 822. The purpose of the scaling circuit 820 is explainedbelow, but its action results in conversion of the commanded motorcurrent I_(c) into commanded generator current I_(G). Error signalgenerator 822 generates an error signal by subtracting a feedback signalfrom a signal path 824, representing the sensed output current of theinternal-combustion-engine/generator (generator), from the commandedgenerator current I_(G). The error signal produced by error signalgenerator 822 is applied to a loop compensating filter, which may be asimple integrator, to produce a signal representative of the commandedspeed of the auxiliary source 16 of electrical energy, more specificallythe diesel engine 18. The diesel engine 18 drives the electricalgenerator 22, to produce alternating output voltage for application byway of power conductors 832 to inverter 28 of FIG. 1. A current sensorarrangement illustrated as a circle 834 is coupled to the outputconductors 832 for sensing the generator current. Blocks 822, 826, 18,22, and 824 of FIG. 8 together constitute a closed feedback loop whichtends to make the output current of generator 22 equal to the magnitudecommanded by the control signal I_(G) applied to the error generator.Loop compensator 826 is selected to prevent the speed of the dieselengine from changing too rapidly, which might undesirably result in anincrease in emission of pollutants,

As so far described, the arrangement of FIG. 8 produces a signal I_(c)for commanding the traction motor current for control of the motion ofthe vehicle, and also produces a signal I_(G) which commands the currentof the auxiliary generator 22. In FIG. 8, a signal representing adesired state of charge (SOC) of the traction battery is received at thenoninverting input port of a summing circuit 850. A signal representingthe current state of charge is received at the inverting input port ofsumming circuit 850 from a battery state-of-charge (SOC) determiningblock 852. SOC block 852 receives signals representative of batteryvoltage, battery temperature, and battery currents. In general, thestate of charge of a battery is simply the time integral of the net ofthe input and output currents. SOC block 852 integrates the net amperesof current to produce ampere-hours of charge. Summing circuit 850produces, on a signal path 854, an error signal which represents thedifference between the desired or commanded state of charge of thetraction battery and its actual state of charge, to thereby identify aninstantaneous surfeit or deficiency of charge. The error signal isapplied to a loop compensating filter 856, which integrates the errorsignal, to produce an integrated error signal. The integrated errorsignal changes slowly as a function of time. The integrated error signalacts on block 820 by way of a limiter 858. More particularly, theintegrated error signal, when applied to scaling block 820, selects thescaling factor by which the commanded motor current I_(c) is scaled tomake it into the commanded generator current. Limiter 858 merely limitsthe integrated error signal from block 856 so that the range of scalingfactors of scaling block 820 is limited to the range between zero andone (unity). Thus, the commanded generator current I_(G) can never begreater than the commanded traction motor current I_(c), but can be lessaccording to the scaling factor commanded by the limited integratedsignal from limiter 858, and the commanded generator current I_(G) canbe as low as zero current.

The desired state of charge of the traction battery is a charge levelwhich is less than full charge, so that regenerative braking can beapplied without danger of damaging the traction battery due toovercharging. Thus, the set-point of the desired SOC is a charge lessthan full charge. The operation of the arrangement of FIG. 8 can beunderstood by assuming that the normal state of the output of theintegrator in loop compensating filter 856 is 0.5 "volts," halfwaybetween the 1.0 volt maximum and the 0.0 volt minimum permitted bylimiter 858. The value of the integrated error signal (as limited bylimiter 858) may be viewed as a multiplying factor by which scalingcircuit 820 scales the commanded traction motor current, so that anintegrated error signal having a value of 1.0 causes the commandedtraction motor current I_(c) to be transmitted at full amplitude byerror signal generator 822, while a value of 0.5 would result in themagnitude of the commanded generator current I_(G) to be exactly half ofthe magnitude of the commanded traction motor current I_(c). Inoperation of the vehicle under the control of the arrangement of FIG. 8,as the traction battery exceeds the desired state of charge, errorsignal generator 850 subtracts a large signal value representing a highstate of charge from the set-point value, thereby producing a differenceor error signal having a negative polarity. The integrator in loopcompensating filter 856 integrates the negative-polarity signal, whichtends to "reduce" or drive negative the net integrated signal at theoutput of loop compensating filter 856 away from its "normal" value of0.5 volts, possibly down toward 0.3 volts, as an example. Since a valueof 0.3 volts of the integrated error signal lies within the permittedrange of limiter 858, the integrated error signal simply flows throughlimiter 858, to control scaling circuit 820 in a manner which causes thecommanded traction motor current I_(c) to be multiplied by 0.3, ratherthan the "normal" 0.5, to produce the commanded generator current I_(G).Thus, a state of battery charge greater than the desired set-pointresults in reduction of the average output of the generator. In the samemanner, if the charge state of the traction battery is lower than thedesired set-point, the signal applied from block 852 of FIG. 8 to theinverting input port of error signal generator 850 becomes smaller inmagnitude than the signal representing the desired SOC, which results ina positive value of error signal at the output of error signal generator850. The integrator associated with loop filter 856 integrates itspositive input signal to produce an integrated output signal which tendsto increase above its "normal" value of 0.5 volts, to a value of, forexample, 0.8 volts. Since this value is within the values acceptable tolimiter 858, the 0.8 volt integrated error signal is applied to scalingcircuit 820 without change. The 0.8 volt integrated error voltage causesscaling circuit 820 to multiply the signal representing the commandedtraction motor current I_(c) by 0.8, so that the commanded generatorcurrent I_(G) is greater than previously. The net effect of the decreasein traction battery charge to a value below the set-point is to increasethe average output power from generator 22, which should tend toincrease the traction battery charge level. Those skilled in the artwill understand that the "normal" value of integrated error signalreferred to above does not actually exist, and is used only to aid inunderstanding the operation of the control system.

Thus, according to an aspect of the invention, a method (FIGS. 7a, 7b,8) for operating a hybrid electric vehicle (10) which derives at leastsome of its tractive effort from electric batteries (20) includes thestep (512, 514, 618, 620) of providing energy from a traction battery(20) to a traction motor (40) in at least one operating mode of thehybrid electric vehicle (10), and, from time to time, dynamicallybraking the vehicle (322, 324, 418, 420). The steps of the methodinclude the step (312, 314) of returning to the batteries (20) at leasta portion of the energy made available by the dynamic braking, andcharging the batteries (20) from an auxiliary source (16) of electricalpower during those intervals in which the dynamic braking is notperformed, with the charging being in a "normal" amount, which normalamount may vary in accordance with some control law, in a mannersuitable for normal operation of the vehicle (10). In this aspect of theinvention, the batteries (20) are charged from the auxiliary source (16)of electrical power during those intervals in which dynamic braking isbeing performed, at a rate reduced from the "normal" amount of charging.In another improvement of this aspect of the invention, the step ofcharging the batteries (20) from an auxiliary source (16) of electricalpower includes the step of charging the batteries (20) from anelectrical generator (22) driven by an internal combustion engine (18),which may be a diesel engine. A fuel cell (24) can be used instead of anengine-generator combination.

What is claimed is:
 1. A method for operating a hybrid electric vehiclewhich derives at least some of its tractive effort from electricbatteries, comprising the steps of:providing energy from a tractionbattery to a traction motor/generator in at least one operating mode ofsaid hybrid electric vehicle; from time to time, dynamic braking saidvehicle using said motor/generator, and returning to said batteries atleast a portion of the electrical energy made available by said dynamicbraking; during those intervals in which said dynamic braking is notperformed, charging said batteries from an auxiliary source ofelectrical power separate from said motor/generator, said charging beingin an amount suitable for normal operation of said vehicle; and duringthose intervals in which dynamic braking is being performed, chargingsaid batteries from said auxiliary source of electrical power, at a ratereduced from said amount.
 2. A method according to claim 1, wherein saidstep of charging said batteries from an auxiliary source of electricalpower includes the step of charging said batteries from an electricalgenerator, separate from said motor/generator, driven by an internalcombustion engine.
 3. A method according to claim 2, wherein said stepof charging said batteries from an electrical generator includes thestep of charging said batteries from a generator driven by a dieselengine.
 4. A method according to claim 1, wherein said step of chargingsaid batteries from an electrical generator includes the step ofcharging said batteries from a fuel cell.
 5. A hybrid electric vehicle,comprising:at least one traction drive element contacting the supportsurface: a traction electrical battery; a traction motor/generatorcoupled to said traction element for transfer of mechanical energytherebetween; an auxiliary electrical source separate from said tractionmotor/generator; a controllable electrical interface electricallyconnected to said battery, said motor/generator, and said auxiliaryelectrical source; and control means coupled to said controllableelectrical interface, said battery, said motor/generator, and saidauxiliary electrical source, for, (a) in a first mode of operation ofsaid hybrid electrical vehicle, providing electrical energy to saidmotor/generator, for causing said motor/generator, operating in a motormode of operation, to drive said traction drive element in order topropel said vehicle, (b) in a second mode of operation during whichregenerative braking is not performed, providing electrical energy fromsaid auxiliary source to said battery in an amount established bycontrol laws depending, at least in part, upon the condition of saidbattery, and (c) in a third mode of operation, dynamically braking saidvehicle by operating said motor/generator as a generator and returningto said battery at least a portion of the electrical energy resultingfrom said braking of said vehicle, and in said third mode of operationproviding electrical energy from said auxiliary source to said batteryin an amount less than that established by said control laws in theabsence of said dynamic braking.